1. Field of the Invention
The invention relates to a centrifugal compressor.
2. Discussion of Background
Contactless seals, in particular labyrinth seals, are widely used for sealing rotating systems in turbomachine construction. A high frictional loss occurs in the separating gap through which fluid flows between the rotating and stationary parts because of the boundary layers formed in the flow. This causes heating of the fluid in the separating gap and therefore produces heating of the components surrounding the separating gap. The high material temperatures cause a reduction in the life of the corresponding components.
A centrifugal compressor with a labyrinth seal arranged on the rear wall of the compressor impeller in the separating gap between the compressor casing and the compressor impeller is known from EP 0 518 027 B1. Because of the high pressure at the outlet of the compressor impeller, leakage air can penetrate into the annular space between the rotating wall and the stationary wall of the compressor casing. In order to avoid both this and also the associated heating of the components surrounding the separating gap, a cold gas at a higher pressure than that at the outlet of the compressor impeller is introduced into the separating gap. For this purpose, an additional annular space is arranged within the labyrinth seal and is connected to an external gas feed. The cold gas flows through the wall of the compressor casing into the labyrinth seal and then impinges on and cools the rear wall of the compressor impeller. When it impinges on the rear wall, the gas is divided and mainly flows radially inward and outward through the individual sealing elements of the labyrinth seal. The partial flow directed radially outward, in particular, is intended to prevent hot compressor air from the outlet of the compressor impeller from flowing through the separating gap.
Despite special ancillary components, which make the centrifugal compressor more expensive, the cooling effect of such a solution cannot be considered optimum. The fact is, rather, that during the feed of the cold gas, mixing initially occurs between the partial flow directed radially outward and the boundary layer forming on the rear wall of the compressor impeller. In addition, this partial flow has to do work against at least one sealing element of the contactless seal which, in addition to the worse cooling effect, also causes greater friction on the rear wall and therefore greater mechanical losses.